There were a few acronyms in the past that actually had official meaning such as GTS(Giga-Texel Shader), but none of the current have any significance beyond simply being model designators.
A few could stand for common acronyms such as SE(special edition), LE(light edition), XT(eXtended technology). I always thought MX was chosen in honor of James Maxwell. Can't think of anything computer related for GT, so that probably came from the marketing department.